The present invention relates to an integrated circuit, and, more particularly, to a low voltage bandgap reference manufactured using a deep sub-micron CMOS process having a current complementary to absolute temperature sub-circuit coupled to provide a current substantially constant over temperature.
Various systems, such as analog-to-digital converters (ADC), digital-to-analog converters (DAC), temperature sensors, measurement systems and voltage regulators use bandgap reference circuits to establish the accuracy of the system. Bandgap reference circuits provide local reference voltages of a known value that remains stable with both temperature and process variations. As such, the bandgap reference circuit provides a stable, precise, and continuous output reference voltage for use in various analog circuits. A known bandgap reference circuit derives its reference voltage by compensating the base-emitter voltage of a bipolar transistor VBE for its temperature dependence (which is inversely proportional to temperature) using a proportional to absolute temperature (PTAT) voltage. With reference to FIG. 2, the difference between the base-emitter voltages, VBE1 and VBE2 or xcex94VBE, of two transistors that are operated at a constant ratio between their emitter-current densities forms the PTAT voltage.
The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size. Thus, the basic PTAT voltage xcex94VBE is given by:
xcex94VBE=VBE1xe2x88x92VBE2xe2x80x83xe2x80x83(1)
xcex94VBE=(kT/q)In(J1/J2)xe2x80x83xe2x80x83(2)
where k is the Boltzmann""s constant, T is the absolute temperature in degree Kelvin, q is the electron charge, J1 is the current density of a transistor T1, and J2 is the current density of a transistor T2. As a result, when two silicon junctions are operated at different current densities, J1 and J2, the differential voltage xcex94VBE is a predictable, accurate and linear function of temperature. Consequently, the output current Iout2 is proportional to absolute temperature since Iout2=xcex94VBE/R2. In some applications, however, to better control power consumption, a current substantially independent of temperature is desirable.
In an effort to provide a reference voltage and current that is constant and substantially independent of temperature, a current source that provides a current complementary to absolute temperature (CTAT) is necessary, wherein the PTAT current from the bandgap reference circuit shown in FIG. 2 and the CTAT current are combined. A temperature independent reference current is provided when the PTAT current, that increases with temperature, and the CTAT current, that decreases with temperature are summed together. If the two slopes of both currents, PTAT and CTAT, are equal in magnitude but opposite in sign, the sum will be independent of temperature. This constant current is applied to a resistor to create a constant voltage.
Conventionally, a CTAT current is provided using current that is proportional to the base-emitter voltage of a bipolar transistor VBE for its temperature dependence which is inversely proportional to temperature. The current source shown in FIG. 1 follows this approach. In processes where the gain xcex2 of the bipolar device Q1 is greater than 50, the base current of the bipolar device is ignored. Thus, the output current Iout1 equals VBE/R1, where VBE is the base emitter voltage of bipolar device Q1. Since the base emitter voltage VBE includes a negative temperature coefficient, the output current Iout1 represents a CTAT current. In a CMOS digital process such as Texas Instrument""s (copyright) 1833c05 process, however, the gain xcex2 of bipolar device Q1. is less than 10. As such, the base current IB of the bipolar device Q1. cannot be ignored. Thereby, the total output current Iout1 equals the sum [(VBE/R)+IB]. Thus, the conventional CTAT current source will not provide a CTAT current in a CMOS digital process.
Another approach that provides a current that is temperature independent may include an external resistor to set a temperature independent bias current. Although the external resistor has an adjustable value, most preferred implementations require that all the components be included on the chip.
Another popular approach is to apply a temperature independent reference voltage Vref to a resistor to generate a temperature independent current. Since the resistor""s temperature coefficient cannot be compensated, the output current becomes temperature dependent. This design, however requires an additional buffer stage.
Thus, a need exists for a current source that provides a CTAT current void of bipolar transistor base current, regardless of whether it is implemented in a CMOS digital process or not. This current source must not be a complex circuit requiring an additional buffer stage.
To address the above-discussed deficiencies of current sources that provide CTAT current, the present invention teaches a current source that provides a current CTAT void of bipolar transistor base current, regardless of whether it is implemented in a CMOS digital process or not. This current source does not require an additional buffer stage.
A control circuit according to the present invention includes a bandgap reference for providing a PTAT current connected a first current mirror to generate a current proportional to the PTAT current. A novel complementary to absolute temperature (CTAT) current source in accordance with the present invention connects to the first current mirror such that the current proportional to the PTAT current and the CTAT current are summed together to provide the current that remains substantially constant over temperature.
This CTAT current source includes a first bias current source which connects to a first resistive circuit and a first subcircuit portion. The first subcircuit portion, including a first bipolar transistor, generates a current proportional to the base emitter voltage of the first bipolar transistor and the base current of the first bipolar transistor. A second bias current source connects to a second resistive circuit and a second subcircuit portion. The second subcircuit portion, including a second bipolar transistor, generates a current proportional to the base current of the second bipolar transistor. A second current mirror connects between the first subcircuit portion and the second subcircuit portion to subtract the base current from the first subcircuit portion. A third current mirror connects between the second current mirror and the first current mirror to provide the current that remains substantially constant over temperature.